The advent of Ultra Large Scale Integrated (ULSI) circuits have allowed semiconductor manufacturers worldwide to fabricate semiconductor devices to extremely compact dimensions. The formation of semiconductor devices involves the processes of deposition, lithography, etching and isolation between layers. In order to fabricate integrated circuits (ICs), devices isolated from one another must first be formed in the silicon substrate. With the continuous development of fabrication technology for semiconductor devices, the development of a high density wafer is being carried out at a brisk pace.
Certain applications of ULSI include the use of a microwave device for generating microwave signals. Up to now, a micro inductor, base on has been the choice for acting as one of the elements of the microwave device, because it needs only simple fabrication process. Generally speaking, the such a device includes a micro inductor and a ground plate isolated by insulator, and microwave device can be made by integrated circuits technologies.
In order to reduce the substrate dielectric loss, as shown in FIGS. 1A, 1B, a membrane structure inductor has been used. FIG. 1A shows a cross section view of a conventional membrane structure inductor. The device includes a semiconductor substrate 2 having a cavity 4 formed therein. A ground plate 6 is connected to the lower surface of the substrate 2. An insulator layer 8, such as silicon nitride, is formed on the upper surface of the substrate 2 for isolation. An inductor pattern 10 is patterned on the insulator layer 10 using lithography and etching technologies. A metal layer 12 is used as an airbridge to connect to the inductor pattern 10 using via holes 14. The via holes 14 are typically formed in an isolation layer 16. FIG. 1B is a top view of the inductor pattern 10. It can be seen that an inductor 10 is formed in the area of the membrane window 18.
However, the formation of the membrane is very difficult, and the yield of the device is hard to control. Further, the process of the membrane inductor is not compatible to standard CMOS process.